Self-Healing of Medium-Strength Concrete Using Paenibacillus polymyxa and Calcium Carbonate: Assessment of Crack Closure and Mechanical Recovery for Vulnerable Housing

Concrete infrastructure in coastal regions is prone to premature degradation due to crack formation under aggressive environmental exposure. Conventional repair methods remain costly and often ineffective. This study evaluates a biomineral self-healing system incorporating Paenibacillus polymyxa spores and calcium carbonate (CaCO3) to improve the durability and mechanical performance of medium-strength concrete with a design compressive strength of 21 MPa, intended for vulnerable coastal housing. A full factorial experimental program was conducted using three bacterial concentrations (1.0%, 1.5%, 2.0% of mixing water volume) and three CaCO3 dosages (3%, 5%, 7% as cement replacement). Specimens were pre-cracked under compressive loading, exposed to a simulated coastal environment, and monitored for 28 days. The optimal formulation (2% bacteria + 5% CaCO3) yielded an 8.8% increase in compressive strength and a 24% increase in flexural strength compared with the control. Crack width reduction reached up to 0.23 mm (65.7%) under wet curing, with effective sealing observed for cracks ≤ 0.5 mm. Recovered compressive strength after healing reached 17.3 MPa, equivalent to 71% of the design strength. These findings demonstrate the potential of P. polymyxa as a viable non-ureolytic agent for self-healing concrete, offering a simple and scalable strategy to extend service life in resource-limited coastal regions while supporting Sustainable Development Goals 9 and 11.